This invention relates to modulator/demodulator (modem) apparatus and more particularly to multirate wire line modem apparatus.
The wide proliferation of data transmission over telephone lines and the like has created a need for versatile modem apparatus capable of determining the highest rate at which data is available and accepting or transmitting the same at the highest corresponding rate available. Variable rate modem apparatus is preferred since through the use of this form of modem apparatus the highest rate of the transmitter and/or receiver may be accommodated any time the receiving and/or transmitting station can operate at this rate while lesser rates are available for stations having lesser capability. Thus the instant invention is directed to multirate wire line modem apparatus wherein, according to a preferred mode, the multirate modem apparatus set forth may operate at either 2400 bits per second (bps) or 4800 bps in either transmission or reception modes.
While data rates have upper limits of transmission which under all circumstances are imposed by the transmitting and receiving stations, the highest rates available are frequently foreclosed by the quality of the transmission medium employed. Thus where telephone lines are employed the quality of the line will often limit the maximum data rate available. When voice is involved the quality of the telephone lines is not highly significant. However, here full duplex modes of operation are highly desirable. Furthermore, for voice applications, full duplex operation with the same data rate in each direction is not only desirable but yields highly secure modes of communication which are becoming increasingly important.
One technique for achieving full duplex operation for voice and data transmissions has involved band splitting. This however while compensating for the quality of the telephone lines employed for transmission effectively halves the data rate.
The present invention recognizes that echo cancellation techniques may be employed to improve the apparent quality of a pair of telephone lines utilized for full duplex transmission of voice and data. Furthermore, through the use of echo cancellation employing training for both automatic gain control and the echo cancellation imposed, telephone lines of adequate quality for voice may often be successfully employed for full duplex transmission and reception of data, particularly when such data is accompanied with sufficient protection to allow training to be completed prior to actual data transmission.
The instant invention also recognizes that the modem function may be integrated into a line system function to markedly reduce the system hardware costs involved. This integration yields multirate modem apparatus providing function control, echo cancellation and full duplex modes of operation utilizing a single pair of wires.
The multirate wire line modem apparatus set forth is configured so that the same is controlled by an input/output processor (IOP) which acts as a system controller and as such controls a pair of digital signal processors (DSP 1 and DSP 2). DSP 1 contains and controls the transmission functions of the modem apparatus together with such ancillary functions as echo canceling, automatic gain control and tone detection. DSP 2 controls the receiver functions of the modem.
The IOP, DSP.sub.1 and DSP.sub.2 are separately programmed so that the same may in fact act as parallel processors which, once instructed, act in an essentially independent manner. The multirate modem apparatus set forth provides a tone detection and echo cancellation. Reception is performed employing a phase-locked loop function and an adaptive equalizer. The apparatus is capable of tracking a transmitted symbol rate for a symbol-frequency offset of up to 100 parts per million to achieve receiver synchronization. Automatic gain control circuitry in the form of hardware is controlled by software generated control signals to enable input level ranges of from 0 DBM to -40 DBM to be processed. Negative logarithm functions are employed in the automatic gain control circuitry to achieve adequate range in single precision arithmetic. Thus, in this manner, multirate wire line modem apparatus which is capable of communicating with transmitting and receiving stations at the highest rate available is achieved.
Therefore, it is a principal object of the present invention to provide improved multirate wire line modem apparatus.
Various other objects and advantages of the present invention will become clear from the following detailed description of an exemplary embodiment thereof and the novel features will be particularly pointed out in conjunction with the claims appended hereto.